1. Field of the Invention
This invention relates to a high voltage power supply. It pertains more particularly to a transformer and rectifier set that converts three-phase a.c. energy to a range of d.c. voltages at the full power rating of the primary coil and core. The device will produce a low ripple voltage source that is particularly well suited for energizing electron beam generators used for welding, heat treating and other metal working processes.
2. Description of the Prior Art
Electron beam welding and heat treatment of metals, which includes surface hardening, vacuum melting, and degassing is performed with an electron beam generator whose power source provides between 30 kV and 200 kV d.c. Whether a particular metal target will melt when struck by an electron beam moving across its surface depends on the magnitudes of the beam power and the power density. If the power density is low, the beam energy can be carried away by heat conduction so that no melting occurs. If the power density exceeds a critical minimum level instantaneous melting will occur and only a negligible amount of the energy will be lost by heat conduction. At an even higher threshold of power density, instantaneous evaporation will occur. At intermediate power densities some condition between these extremes will be encountered.
A workpiece cut or welded with an electron beam will look drastically different if the beam has barely enough power to cause melting compared to the result produced by a higher power beam capable of producing instantaneous melting. The melting rate depends on welding speed and beam power density; the power density of an electron beam in air and in metal vapor depends upon the distance it has travelled. Therefore, the depth or thickness through which the beam can cut or weld depends on the speed at which it passes over the surface of the metal target, the beam power density and the distance of the beam source from the workpiece. A lower power beam will not make the same cut or produce the same weld quality as a high power beam simply by moving it slower across the workpiece. Thus, when supplied with power of large magnitude an electron beam can make narrow and deep welds in air with profiles approaching those of welds made in a vacuum.
Electron beam guns from which the electron beam is directed and focused on the workpiece are supplied with d.c. power. When the d.c. power source is not steady but fluctuates about some mean d.c. voltage, the focal point of the electron beam will correspondingly vary from its target position on the workpiece. It is crucial to the correct, predictable operation of an electron beam welding device, therefore, that the power supply provide d.c. voltage with little or no ripple.
The voltage source that powers electron beam guns should have the capacity to produce a range of voltages whose magnitude varies depending on the application. For example, a power source for delivering large magnitudes of power at low voltage is required for heat treatment of large workpieces because rapid energy input over the full surface area will assure rapid self-quenching as the heat is conducted through the workpiece away from the heat treated surface. The power source for use where self-quenching is essential will deliver perhaps 20 to 60 kW at 30 kV. Larger magnitudes of beam power at higher voltages, typically 60 kW or more at 175 kV, are advantageous for electron beam welding in air. Such a power source will produce a beam that heats the air and metal vapors in the space between the beam source and the workpiece to enormous temperatures. This heating produces a rarefaction of the gases and vapors that reduces beam scattering effects. Therefore, the power density of the electron beam can be maintained at maximum levels even though the distance from the beam source to the workpiece target is large.
The power supply for an electron gun capable of welding, heat treating, melting and degassing of metals must provide a wide range of voltages at the full power rating of the device. Preferably the power source will produce the voltage range up to the rated power of the device without having to alter the configuration of the power source.
U.S. Pat. No. 3,418,526 discloses a compact high voltage power supply suitable for an electron beam welder. The supply includes a network for converting voltage received from a motor-generator set to produce an output voltage of 150 keV. The network is enclosed in a container which is filled with a gas of high dialectic strength. The voltage converting network includes a step-up transformer to convert the generated voltage to an intermediate voltage, a voltage multiplying network composed of an a.c. capacitor chain on the input side and a d.c. capacitor chain on the output side and a voltage rectifier chain connected in a voltage multiplying relationship with the capacitors.
U.S. Pat. No. 3,914,575 describes a power supply device for the operation of a gas discharge container used for the treatment of metallic workpieces. The device includes a transformer and rectifier for producing operating voltage for the alternating or three-phase current supply with continuous regulation of operating voltage.